During the manufacture of fiber suspensions, undesired coarse particles, such as, for example, undefibered material, bark, knots and the like, are present in the suspension as a result of incomplete manufacturing processes. In addition, other impurities, which are both light and heavy, such as plastics, sand and scrap, can also be found in the suspension. In particular, fiber suspensions of slushed return fibers contain great amounts of foreign impurities. Heavy impurities, such as stones, sand and glass, plaster and wire clips, and light impurities, such as certain plastics, agglomerated glue lumps ("stickies") etc., can cause interruptions in the screening process. It is, therefore, desired to separate them at an early stage of the screening process. For this purpose, special devices, for example knot screens or refiners, can be arranged before the screen in order to eliminate the coarse impurities or reduce their size. Light impurities can also be separated by special devices before the screen. It is also possible to separate the coarse impurities by a first screening step in the screening arrangement or to separate scrap and heavy particles when the suspension enters the screen, and other impurities during the fine screening process. In the last mentioned case, the screening process can be disturbed by the impurities, as mentioned above.
During the screening of pulp suspensions it is also desired to have a high pulp concentration, for example, about 3 to 5%, and a low reject draw-off in order to achieve a high production capacity and to avoid unnecessarily large liquid transport in the screening system. High concentration and low reject draw-off, however, imply greater difficulties in separating the impurities from the pulp.